Another relative question if you allow. Could you confirm to me that after having switched to the Cloud9 plan for the 3.x version, if we migrate the app to 4.x then we will be allowed to downgrade to the free plan at any time? And in this case, only month consumed is billed?

In 3.x there is no such thing as the "free plan" anymore, as a result, you would not be able to downgrade. When you migrate to 4.x, you can simply delete your app in 3.x to avoid any future billing there.